ssh: use upstream SSH_OPENSSL_VERSION macro

With the upgrade to OpenSSH 6.7p1 in commit a0ee8cc636cd we replaced
WITH_OPENSSL ifdefs with an OPENSSL_VERSION macro, later changing it
to OPENSSL_VERSION_STRING.

A few years later OpenSSH made an equivalent change (with a different
macro name), in commit 4d94b031ff88. Switch to the macro name they
chose.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation